The invention is directed broadly to a matching circuit for delivering radio frequency electromagnetic energy to a plasma. More particularly, the invention is directed to such a matching circuit adapted for use with a power supply in applications such as sputter coating, sputter etching and other plasma or glow discharge processes commonly used in the thin film industry.
Sputtering devices or gas plasma devices driven by radio frequency (RF) electromagnetic energy are known in the prior art. Such devices are frequently used in the thin film industry, for example, to manufacture semiconductors. All such devices suffer from the common problem that a fixed impedance or variable impedance radio frequency generator must be coupled to the variable impedance load of a plasma disposed within a vacuum chamber. In the prior art, impedance matching circuits are provided which are quite similar to antenna matching circuits used in the broadcast industry. Such prior art matching circuits suffer from problems that stem both from the operational theory of the circuit as well as the design of the components used to carry out the impedance matching function.
In prior art impedance matching circuits used in this application, only two variable capacitors are provided and the process of changing the voltage ratio between the generator and power supply changes the tuning so that when one servo loop moves, the other servo loop must move in response thereto to achieve a match. The result is an iterative process or a phase space spiral approach to impedance match rather than a direct approach to impedance match. A spiral rather than a direct approach to impedance match is an inherently slower and imprecise technique. In a gas plasma application it is very important to achieve impedance matching as quickly as possible to achieve a consistent process rate such as consistent deposition of the target material in a sputtering operation. Thus, in the thin film industry there is a need for an impedance matching circuit for a radio frequency generator having better speed, and improved dynamic accuracy to the match.
In general, the prior art has used linear vacuum capacitors with a gear motor, gear rack, or lead screw-type drive and a bang-bang servo drive without variable separation to accomplish tuning. Linear gear driven capacitors are not readily adapted for high speed operation or synchronous movement when multiple capacitors are required. Another problem with prior art linear vacuum capacitors results from the need to make electrical contact through moving components such as bearings, bellows and the like.
Other problems with prior art impedance matching circuits stems from the fact that the power level in the circuit is relatively high, on the order of several kilowatts. In addition to the power level being high, the plasma load is very electrically reactive so that in addition to the high power flow there is a reactive power flow or energy circulation between the tuning elements which is quite large. In some cases, the power circulation between the tuning elements of the matching circuit can be over 10 times the real power flowing into the plasma load. Thus, it is quite desirable to keep losses at a minimum. Prior art inductances used in this application are generally of a solenoidal coil type which has very high end losses. Since power levels are high, a relatively large diameter conductor is often necessary to accommodate power. However, large diameter conductors also have large eddy current losses. When cylindrical-shaped inductors are wound with strap or flat conductor, the end loss problem is exacerbated because the end of the strap looks like a shorted turn. When toroidal-shaped inductors are wound with strap conductor variable and inevitably large insulating gaps on the periphery of the toroid also present substantial losses.